Drilling of oil and gas wells typically involves the use of several different measurement and telemetry systems to provide data regarding the subsurface formation penetrated by a borehole, and data regarding the state of various drilling mechanics during the drilling process. In measurement-while-drilling (MWD) tools, for example, data is acquired using various sensors located in the drill string near the drill bit. This data is either stored in downhole memory or transmitted to the surface using assorted telemetry means, such as mud pulse or electromagnetic telemetry devices. Such sensors require electrical power and, since it is not feasible to run an electric power supply cable from the surface through the drill string to the sensors, the electrical power is often obtained downhole.
In some cases, for instance, the sensors may be powered using batteries installed in the drill string at or near the location of the sensors. Such batteries, however, have a finite life and complicate the design of the drill string by requiring a sub/housing that houses the batteries and associated sensor boards. Moreover, batteries take up a substantial amount of space in the drill string and can therefore introduce unwanted flow restrictions for circulating drilling fluid. In other cases, the sensors may be powered using an electrical power generator included in the drill string. For instance, a typical drilling fluid flow-based power generator employs a rotor shaft having multiple rotors extending radially therefrom. The rotors are placed in the drilling fluid flow path to convert the hydraulic energy of the drilling fluid into rotation of the rotor shaft. As the rotor shaft rotates, electrical power may be generated in an associated coil generator. In other applications, the rotational energy of the rotor shaft may be transmitted to various downhole devices, if desired.